1. Field of the Invention
The present invention relates to active vibration and noise control systems. More particularly, the present invention relates to the active vibration control, wherein the control signal sent to the actuator(s) is optimized, on-line.
2. Description of the Prior Art
Vibration and noise control, although an old problem, has become an increasingly important issue in recent times, especially in the industrial environment. This is mainly due to the increase in usage of machinery in virtually all aspects of our lives, especially with the recent introduction of office automation. Today, one major factor in evaluating any product is its level of vibration and noise generation. Space platforms, electronic computer equipment, high precision manufacturing machinery, portable generators, appliances, automobiles, medical equipment, etc. are all examples of structures that may experience vibration and noise generation problems.
Formerly, vibrations were controlled using passive vibration control systems. However, passive vibration control mechanisms suffer in that they are incapable of handling variable speed drive systems and random excitations, especially at low frequencies. There have been a number of active control techniques developed. Many have been adapted to compensate for the problem of noise control, while only a few have addressed the issue of vibration control.
Much of the former work in the active vibration control area can be divided into three main groups: (1) those that require a synchronizing or "synch" signal and use adaptive filtering techniques to synthesize the wave; (2) those that require direct measurement of the excitation source and may or may not use filtering to compensate for other dynamics such as the actuators; and (3) those that perform off-line design analysis and assume, erroneously in many cases, that signals and systems do not change with time.
One problem with systems requiring a synch signal is that those systems have no mechanism to compensate for nonrepetitive (random) vibrations or noise sources. As to the second group above, systems which require direct measurement of the excitation (a direct feedforward signal) may not be possible or may be impractical to implement, due to the difficulty in locating a sensor at an excitation point. Further, equipment useful for direct measurement of an excitation source may potentially increase the cost of the system.
Further, one problem inherent with any system that requires off-line modeling and design is that real systems change over time, especially if inputs to the system are time varying. A system designed off-line cannot take changes in the system into account. As such, none of the above-listed approaches provides an economical, practical and efficient solution to the problem of vibrations in a system.
Additionally, many prior art systems are slow due to the time intensive calculations used to obtain a transfer function used by the controller and actuators to control or counter-act vibrations. One example of this type of prior art is U.S. Pat. No. 4,490,841 which calculates Fourier transforms when in operation.
One important point to note about the above-listed approaches to active vibration control is that, although vibration and noise cancellation is a "control" problem, the above listed methods attempt to approach the subject of active vibration control from a "signal processing" and/or "filtering" view point. Thus, these techniques may not have a guaranteed stability.
There is a need for an active vibration system that can be adapted quickly on-line to compensate for vibration, due to both random and repetitive excitations, in a system and which does not have the above-described drawbacks of the prior art. Further, there is a need for an active vibration control system which approaches the vibration control application from a "control" problem point of view.